1. Field of the Invention
The present invention relates to a load driving device, and more particularly, to a load driving device including an output transistor that controls power supply to a load.
2. Description of Related Art
Semiconductors for power supply have been widely employed as load driving devices that supply power from a power supply to a load. In one field of application, the semiconductors are used to drive actuators or lamps of vehicles.
In such a load driving device, a ground terminal of the load driving device and a ground terminal of the load may be arranged apart from each other. In this case, a voltage difference may be generated between a ground voltage of the load driving device and a ground voltage of the load due to a resistor component of a conductor between the both terminals. In some cases, the load driving device may be mounted in a unit. In this unit, a ground voltage is supplied to the load driving device through a connector. In this case, the supply of the ground voltage to the load driving device may be interrupted due to a defective connector, disconnection of a wire, or the like. In short, the ground terminal of the load driving device may be brought into an open state. In addition, the ground terminal of the load driving device may be supplied with a voltage by a parasitic device.
When the ground terminal is in the open state, there is a possibility that an output transistor having a function of switching between the load and the power supply becomes conductive, although a gate voltage of the output transistor has not reached a sufficiently high level. In other words, the output transistor may become conductive in the state where a resistance value between a drain and a source of the output transistor is high. This causes a problem that the output transistor may be destroyed due to overheating. For this reason, there is a demand for maintaining the output transistor in a non-conduction state when the ground terminal is in the open state.
Further, in the case of using the load driving device for a vehicle or the like, when the load driving device is in a standby state, there is a demand for preventing a wasteful consumption current from occurring, with a standby current on the order of microamperes.
Japanese Unexamined Patent Application Publication No. 2009-165113 discloses a solution for these demands. FIG. 14 is a circuit diagram of a load driving device 1 disclosed in Japanese Unexamined Patent Application Publication No. 2009-165113. As shown in FIG. 14, the load driving device 1 includes a power supply 10, a load 11, a driver circuit 12, a control circuit 13, a back gate control circuit 15, a compensation circuit 16, an output transistor T1, a resistor R10, a power supply terminal PWR, a ground terminal GND1, a ground terminal GND2, and an output terminal OUT. Connections among the components of the load driving device are described in detail in Japanese Unexamined Patent Application Publication No. 2009-165113, so the description thereof is herein omitted.
Now, an operation of the load driving device 1 will be described. First, a mode in which the output transistor T1 becomes conductive in a normal operation is described. In the conduction mode, a discharge transistor MN1 becomes non-conductive when a control signal S2 of low level is applied to a gate of the discharge transistor MN1. Further, a negative-polarity-side voltage VSS (e.g., 0 V) of the power supply 10 is supplied to the ground terminal GND2, and thus a compensation transistor MN7 also becomes non-conductive. Meanwhile, the output transistor T1 becomes conductive when a control signal S1 of high level, which is output from the driver circuit 12, is applied to a gate of the output transistor T1. Accordingly, in the conduction mode, the voltage value of the output terminal OUT is substantially equal to the value of a positive-polarity-side voltage VB of the power supply 10. Further, in the conduction mode, N-type MOS transistors MN5 and MN6 of a second switching portion 15b become conductive, and N-type MOS transistors MN3 and MN4 of a first switching portion 15a become non-conductive. As a result, a voltage of the ground terminal GND2 is applied to a back gate of the compensation transistor MN7. At this time, in the compensation transistor MN7, a terminal coupled to the output terminal OUT serves as a drain, and a terminal coupled to a node A serves as a source.
Next, a non-conduction mode in which the output transistor T1 becomes non-conductive in the normal operation. In the non-conduction mode, the discharge transistor MN1 becomes conductive when the control signal S2 of high level is applied to the gate thereof. Further, the negative-polarity-side voltage VSS (e.g., 0 V) of the power supply 10 is supplied to the ground terminal GND2, and thus the compensation transistor MN7 becomes non-conductive. Meanwhile, the output transistor T1 becomes non-conductive when the control signal S1 of low level is applied to the gate thereof. Accordingly, in the non-conduction mode, the voltage value of the output terminal OUT is substantially equal to the voltage value (e.g., 0 V) of the ground voltage GND1 of the load 11. Further, in the non-conduction mode, the N-type MOS transistors MN3 and MN4 of the first switching portion 15a become non-conductive, and the N-type MOS transistors MN5 and MN6 of the second switching portion 15b also become non-conductive. Thus, the voltage applied to the back gate of the compensation transistor MN7 by the back gate control circuit 15 is 0 V which is the voltage difference between the ground terminal GND2 and the output terminal OUT.
Next, a mode in which the ground terminal GND2 indicates an open state due to a defective wiring connection or the like (defective GND connection mode) is described. Assume that in the defective GND connection mode, the output transistor T1 is non-conductive. Accordingly, the output terminal OUT is 0 V. When the voltage of the ground terminal GND2 becomes higher than 0 V and exceeds a threshold voltage of the compensation transistor MN7, the compensation transistor MN7 becomes conductive, thereby short-circuiting the node A and the output terminal OUT. Thus, the voltage (e.g., 0 V) of the output terminal OUT is supplied to the node A, with the result that the voltage between the source and drain of the discharge transistor MN1 becomes substantially zero. This prevents a leak current from flowing to the gate of the output transistor T1 through the discharge transistor MN1. In other words, the gate voltage of the output transistor T1 does not rise due to a leak current. This allows the output transistor T1 to maintain the non-conduction state. Further, in the defective GND connection mode, the N-type MOS transistors MN3 and MN4 of the first switching portion 15a become conductive and the N-type MOS transistors MN5 and MN6 of the second switching portion 15b become non-conductive. As a result, the voltage applied to the back gate of the compensation transistor MN7 by the back gate control circuit 15 is equal to the voltage of the output terminal OUT. That is, in the defective GND connection mode, the back gate voltage of the compensation transistor MN7 is 0 V. In this case, in the compensation transistor MN7, the terminal coupled to the output terminal OUT serves as the source, and the terminal coupled to the node A serves as the drain.
As described above, the load driving device 1 of the prior art supplies the voltage of the output terminal OUT to the node A by rendering the compensation transistor MN7 conductive, even when the voltage of the ground terminal GND2 increases due to a defective connection of a ground wire. As a result, the load driving device 1 of the prior art brings the potential difference between both terminals of the discharge transistor MN1 to substantially zero, thereby bringing the discharge transistor MN1 into the non-conduction state. This prevents a leak current from flowing to the gate of the output transistor T1 through the discharge transistor MN1. By this operation, the load driving device 1 of the prior art can maintain the non-conduction state of the output transistor T1 even when the voltage of the ground terminal GND2 increases. Consequently, the load driving device 1 of the prior art can prevent heat generation in the output transistor T1 and also prevent breakdown of the output transistor T1. In short, the load driving device 1 of the prior art can improve the reliability at the time of malfunction caused by a disconnection of a ground wire or the like.